Sol-Gel Synthesized Semiconductor Oxides in Photocatalytic Degradation of Phenol

Effectiveness of photocatalytic degradation of phenol in aqueous solution using semiconductor oxides (SO) prepared by a sol-gel method was examined.The physical and chemical properties of synthesized catalysts were investigated by X-ray diffraction (XRD), diffuse reflectance UV-Vis spectroscopy (DRS), and N 2 -adsorption measurements. The optimal conditions of the photocatalytic degradation of phenol using prepared titanium dioxide sample were defined.


Introduction
Heterogeneous photocatalysis on the semiconductors allows achieving complete mineralization of the various classes toxic and biorefractory organic substances [1,2].Recently, the photocatalytic degradation of toxicants became one of the most promising directions of "green chemistry" [3].The use of nanosized SO presents a great interest due to their outstanding optical, magnetic, catalytic, and sensing properties [4,5].The phenolic compounds contained in the wastewater of chemical, petrochemical, and pharmaceutical industries are hazardous carcinogenic and mutagenic pollutants [6,7].Furthermore, the oxidation of these substances in water bodies leads to decrease in dissolved oxygen required for normal functioning of animals and plants.Finding effective methods for the protection of water systems from phenols contamination is an important aim to ensure environmental safety [8,9].
Among SO photocatalysts (PC) high activity have Fe 2 O 3 , WO 3 , ZnO and TiO 2 .Iron oxide polymorphs of hematite (-Fe 2 O 3 ) are nontoxic, cheap, and stable to photocorrosion material intensively absorbs radiation in the range from 295 to 600 nm.The semiconductor properties of -Fe 2 O 3 are the same as WO 3 , which can be seen in the position of band gaps relative to the standard hydrogen electrode.WO 3 has chemical stability in acidic medium and electrolyte solutions as well as photoactivity in the near ultraviolet and blue regions of solar spectrum [10].According to Daneshvar et al. nanosized ZnO is a suitable alternative to TiO 2 due to the band gap energy.Dinda and Icli found that ZnO was as reactive as TiO 2 for the photocatalytic degradation of phenol under concentrated sunlight [11].Figure 1 shows a scheme of the energy levels of the studied semiconductor oxides relative to the standard hydrogen potential [12].Several authors have associated the efficiency of semiconductor photocatalysts with electronic, structural, and morphological properties of the material such as band gap energy, crystalline structure, surface area, particle size [13].
The activity of semiconductor oxides prepared by the solgel methods was investigated under the same conditions for searching of the most effective system in the reaction of the phenol photodegradation.The optimal parameters of the phenol photodegradation on the synthesized PC were defined.

Materials.
All solvents and chemicals used in this work were of analytical grade and were used without further purification.Inorganic (Fe(NO 3 ) 3 ⋅9H 2 O, Zn(NO 3 ) 2 , Na 2 WO 4 ⋅2H 2 O), and organic (Ti(i-OPr) 4 ) precursors for the synthesis of oxides were purchased from Sigma-Aldrich.The homogeneous precipitant urea (CO(NH 2 ) 2 ) and nitric acid (HNO 3 ) were purchased from Merck (Germany).In some cases the pH solutions were varied with NaOH and H 2 SO 4 (Merck, Germany).P25 TiO 2 was purchased from the Degussa Company in Germany and was used as the reference sample.The Millipore Q Millipore system was used for water purification.

Synthesis of Semiconductor Oxides
Powders.Synthesis of PC was carried out by sol-gel method which allows obtaining nanosized metal oxide particles with desired structural and morphological properties [14].
Fe 2 O 3 sample was synthesized similar to the method [15].Solutions of Fe(NO 3 ) 3 and CO(NH 2 ) 2 were slowly added in the heated deionized water with continues stirring.The mixture was heated at 363 K for 5 h to form and aging of Fe(OH) 3 sol.The resulting russet precipitation was dried at 383 K and calcined at 773 K for 2 h in the air.
WO 3 sample was prepared by thermal decomposition of tungstic acid obtained by the sol-gel method [13].In this procedure, sodium tungstate dihydrate was dissolved in deionized water under continuous stirring at 353 K.After total dissolution, concentrated nitric acid was added dropwise to the sodium tungstate solution.The reaction mixture was subjected to the aging process during 40 minutes at 353 K and left for 24 h at 293 K.The resulting pale yellow precipitation was filtered, washed with deionized water, dried at 353 K, and calcined at 773 K during 2 h in the air.
Synthesis of nanocrystalline ZnO powder was performed similar to [16], by alkaline hydrolysis of zinc nitrate in thermoinitiated decomposition of urea.The aqueous solutions of Zn(NO 3 ) 2 and CO(NH 2 ) 2 was heated at a temperature of 363 K and stirred for 24 h.The resulting precipitation was washed, dried at 383 K, and calcined at 773 K for 2 h.
Nanocrystalline TiO 2 particles were synthesized by the hydrolysis of titanium isopropoxide [17] using a modified method [18].Synthesis was carried out under vigorous stirring in excess of 2-propanol.The reaction mixture was gradually heated to 358 K with addition of deionized water to eliminate the intermediate gelation process.The resulting solid precipitation was filtered, dried in the air at 358 K, and calcined at 773 K.
As a reference sample used a titanium dioxide Degussa P25 (TiO 2 P25), obtained by high-temperature gas-phase oxidation of titanium tetrachloride vapors [19].

Photocatalytic Reactor and Experimental
Procedure.Photocatalytic degradation of phenol was carried out in a 0.5 L quartz reactor with a jacket under air bubbling (velocity 50 mL⋅min −1 ) and temperature from 293 K to 323 K.The reaction mixture was agitated with a magnetic stirrer (800 rpm⋅min −1 ).The concentrations of phenol and catalyst were 0.532 mM and 1 g⋅L −1 , respectively.Low-pressure mercury lamp DRB-8 submerged in a quartz casing used as a UV-radiation source with a maximum emission output at 254 nm.Reaction time was 3 hours.The separation of reaction mixture was performed by centrifugation.The effectiveness of photocatalytic process was evaluated relative to the photolysis carried out under similar conditions without catalyst usage.The phenol conversion was determined by the aromatic content recorded by the absorbance of the solution at 270 nm ( 270 ) with a Shimadzu UV-2405 spectrometer and concentration of total organic carbon ( TOC ) measured by Shimadzu TOC-VCSN analyzer.

Results and Discussion
3.1.Photocatalysts Characterization. Figure 2 presents the Xray powder diffraction patterns of the synthesized semiconductor oxides and reference sample TiO 2 P25.
The XRD pattern of Fe 2 O 3 sample shows that all basal reflections in the range of Bragg angles (2) from 10 to 80 characterize of isomorphic hematite phase (-Fe 2 O 3 ), corresponding to the orthorhombic crystal system (JCPDS No. 79-1741).
Investigation of the crystal structure of WO 3 confirms the presence of hexagonal phase (R6/mmm), JCPDS No. 33-1387).Low peak-height indicates the weakly crystallized structure.
The X-ray diffraction pattern of the prepared TiO 2 sample is presented basal reflections (at around 2Θ 25.4, 44.2, and 56.4) corresponding to the titanium dioxide anatase phase [17].
XRD analysis of the synthesized ZnO shows strong and high peaks indicating the high purity and crystallinity.Location of the basal reflections confirms hexagonal structure of ZnO (JCPDS No. 80-0075).Diffuse reflectance spectroscopy allows obtaining information about light absorption range and band gap of the semiconductor [20].Figure 3 depicts diffuse reflectance spectra of the synthesized materials converted in accordance to the Kubelka-Munk function.The energy band gap (  , eV) is determined by extrapolating of the onset of the rising part to -axis (  , nm) of the plots by the dotted line and calculation by the following equation (see [21]): It can be clearly seen from Figure 3 that the radiation absorption by WO 3 sample begins in the visible range at 500 nm.The band gap of WO 3 is 2.55 eV which is consistent with literature data [22].
The optical absorption spectrum of the ZnO sample is represented by a broad and intense band and characterized by a sharp increase of absorption at 400 nm and a slight decrease at shorter wavelengths.
In the DRS of the TiO 2 P25 and synthesized TiO 2 powder the drastic increasing of the light absorption at  = 380 nm corresponding to the energy band gap of pure anatase (∼3.2 eV) can be noted [23].
The surface area and micropore volume of the synthesized materials are defined with nitrogen adsorption-desorption isotherm.Surface parameters and the energy band gap of the SO are shown in Table 1.It is known that -Fe 2 O 3 and ZnO particles obtained by a sol-gel method by means of thermoinitiated decomposition of urea have a relatively large size (2000 and 3000 nm, resp.)[15,16] and a low specific surface area.The synthesized WO 3 sample has a high specific surface area with a small volume share of the micropores.Despite the high dispersion of the synthesized TiO 2 sample, its specific surface area is less than that of the TiO 2 P25 sample because of lower specific volume of the micropores.

Photodegradation of Phenol.
The results of photolysis and photocatalytic degradation study of phenol using prepared semiconductor oxides and TiO 2 P25 samples are shown in Figure 4.
During the photolysis of phenol under UV-C irradiation the appearance of light brown color and increase of the optical density of analyzed solution are observed, which can be explained by the formation of colored intermediates: benzoquinone, hydroquinone, and catechol [6].Incomplete oxidation of phenol confirmed its low mineralization (22%, Figure 4), and indicates necessity of catalytic method usage.
The study of phenol conversion dependence on the SO nature found that the least active are the -Fe 2 O 3 and WO 3 samples (mineralization is 14% and 26% resp.).The increase of optical density of the solution after photocatalysis suggests the formation of colored intermediates.The phenol conversion using ZnO powder was 24% of aromatic content and 52% of TOC.TiO 2 samples showed the highest activity.Application of synthesized sample TiO 2 leads to aromatic content of 79% and TOC of 85% removal.These results are similar to the activity of TiO 2 P25 sample.
-: the reduction of optical density does not occur.
Table 2 shows the phenol aromatic content ( 270 ) and TOC ( TOC ) conversion dependence on the SO nature, as well as the total () and specific () catalytic activity of investigated SO, calculated by the following equation (see [24]): where  is the reaction time (sec); m is the mass of catalyst (g);  BET is the specific surface area data obtained from the BET-model (m 2 ⋅g −1 );   is the catalyst concentration (g⋅L −1 ).The use of -Fe 2 O 3 and WO 3 samples in the reaction of phenol photocatalytic degradation leads to increase of solution optical density through the formation of colored intermediates.These oxides showed the lowest activity because of their low redox potential (Figure 1).In contrast, the usage of ZnO, TiO 2 , and TiO 2 P25 samples reduces aromatic content and total organic carbon.ZnO sample takes an intermediate position among the studied semiconductor oxides by the values of the total and the specific catalytic activity.TiO 2 sample synthesized by a modified sol-gel method has a higher specific activity compared with TiO 2 P25 (9.2 and 8.6 M⋅m −2 ⋅s −1 , resp.) due to predomination of crystal modification of anatase which compared with rutile has a high surface concentration of active catalytic centers [25].
Investigation of the degradation process of phenol was followed by pH measuring of the reaction mixture.In all cases the pH decrease is associated with the formation of shortchain fatty acids [26].
It is found that the specific activity of SO in the reaction of phenol photocatalytic degradation changes in a number of -Fe 2 O 3 < WO 3 < ZnO < TiO 2 P25 < TiO 2 .Therefore, the determination of optimal conditions for phenol photocatalytic oxidation was carried out using the TiO 2 sample synthesized by the modified sol-gel method.
The influence of the catalyst concentration on the phenol conversion in water (Figure 5) showed a maximum efficiency at a concentration of 1 g⋅L −1 .Lower and higher concentrations of TiO 2 sample led to decrease of conversion degree associated with reduction of active sites number and radiation screening effect of TiO 2 particles excess [27].
Effect of initial phenol concentration on its conversion in water is shown on Figure 6.Rise of phenol concentration from 0.266 to 1.596 mM leads to decrease in the conversion degree for two indicators that can be attributed to an increase in the absorption of radiation by phenol molecules more than that of the catalyst particles and the increase of competitive adsorption of − OH on the same surface site of catalyst [27].
Phenol photodegradation efficiency largely depends on the pH value.Studies showed that aromatic content degradation of phenol and reduction of TOC in an acidic medium (pH = 3) are significantly low (37% and 34%, resp.) in comparison with in alkaline medium (pH = 8) (72% and 56%, resp.).Effect of pH on the photodegradation degree of phenol can be caused by changing in the surface charge of semiconductor, phenol chemical transformations in the solution, and carbonate ions formation which are effective scavengers of OH • radicals [27].
The study of photocatalytic phenol conversion dependence on reaction temperature was carried out in range from 293 to 323 K. Maximum conversion (96% of aromaticity and 86% of TOC conversion) in temperature range 303-313 K was observed.
The efficiency evaluation of the synthesized TiO 2 sample in the real conditions was carried out in Kiev tap water [28], Ukraine (Figure 7).The experimental conditions were the same as in the case of semiconductor oxides activity determination.At the beginning of the reaction rise of solution optical density takes place (Figure 7) which can be explained by the formation of intermediates.Further oxidation leads to the destruction of aromatic content and increase of conversion.

Conclusions
The specific activity of investigated semiconductor oxides in the reaction of phenol photocatalytic degradation changes in a number of -Fe 2 O 3 < WO 3 < ZnO < TiO 2 P25 < TiO 2 .It is shown that titanium dioxide synthesized by the modified solgel method is the most effective photocatalyst in this process.The resulting TiO 2 has a higher specific catalytic activity than industrial sample TiO 2 P25.The maximum efficiency of the catalyst usage is achieved in 303-313 K temperature range at TiO 2 concentration of 1 g⋅L −1 , phenol concentration of 0.532 mM, and neutral pH.The presence of electrolytes reduces the efficiency of the process.

Figure 1 :
Figure 1: Energy band gap of investigated semiconductor oxides.

Figure 7 :
Figure 7: The photocatalytic conversion of phenol in tap water using synthesized TiO 2 samples.

Table 1 :
Physical and chemical characteristics of the investigated semiconductor oxides.

Table 2 :
Degree of substrate conversion ( 270 and  TOC ) and total () and specific catalytic activity () of investigated SO in the reaction of phenol photocatalytic degradation in water.